tart pull: try to re-use APFS blocks by cloning the base image

[edigaryev]

To be honest, I'm not a fan of this approach, because it's incredibly hard to measure the real-world impact of this, both in terms of CPU overhead and disk usage savings¹.

The reason being that these variables are highly influenced by the contents of the base VM image (from which we're de-duplicating) and by the contents of the VM image to be pulled, yet we only have a few publicly available.

¹: It feels that in some cases even losses, because we can't skip writing a zero block if the file we're basing off contains some other data in that place.